This invention relates to plasma devices. In particular, it relates to toroidal plasma devices including those referred to as Tokamaks which are toroidal in their design and which include some magnetic coils which are toroidal about the axis of the device.
In order to achieve the design objectives of toroidal plasma devices, it is normally necessary to apply several magnetic fields in several different directions and subject to different types of control. One example of such a device is the Tokamak, a device which generates and confines plasma in a substantially toroidal shape. The combined requirements of confinement and heating lead to the use of several electromagnetic coils to generate particular magnetic fields. One set of coils is wound spirally about the toroidal plasma, enclosing the minor axis of the toroid. This is the so-called toroidal-field coil, referred to here as the TF coil, the main confinement coil of the plasma. It has long been known that a plasma confined only by a TF coil is unstable and that to stabilize such a plasma it is necessary to add one or more coils that are substantially parallel to the midplane of the toroid and coaxial with its major axis. These may be referred to either as poloidal coils or as equilibrium-field coils. They will be referred to here as equilibrium-field (EF) coils. The third type of coil that may be involved in a toroidal plasma is an ohmic-heating (OH) coil that is typically wound to couple by transformer action to the plasma which is then envisioned as a one-turn secondary of the transformer for which the OH coil is the primary. Other systems such as RF induction heating or neutral-beam injection have been proposed to heat toroidal plasmas. Whether or not an OH coil is used, the region in and around the EF coil is one of large magnetic fluxes that in general are varying rapidly over considerable ranges. Fault protection for a toroidal coil such as the EF coil is important to the safe operation of the plasma device and is made more challenging by the presence near that coil of the various magnetic fields. The EF coil is typically carrying large enough amounts of current, of the order of thousands of amperes, so that faults such as shorts from turn to turn or from the coil to a grounded part of the structure can do considerable damage if not interrupted very rapidly. A short from the coil to ground may be detectable in time to minimize further damage by sensing means responsive to the coil current but such a protection would not suffice to guard against damage by shorts from turn to turn of the EF coil.
It is an object of the present invention to provide fault protection for an electromagnet in a toroidal plasma device.
It is a further object of the present invention to provide information about developing electrical faults in an equilibrium-field coil in a toroidal plasma device.
It is a further object of the present invention to provide a sensor of changing magnetic fields that is responsive to changes in the field from one coil in the presence of other changing magnetic fields.
Other objects will become apparent in the course of a detailed description of the invention.